The present disclosure relates to a mode-locked semiconductor laser device and a driving method thereof.
Ultrashort pulse/ultrahigh output lasers are actively used for research in advanced scientific fields using laser light whose pulse time is in attosecond or femtosecond. Moreover, a high output ultrashort pulse semiconductor laser device composed of GaN-base compound semiconductor and whose luminous wavelength is in the 405 nm band is expected as a light source of a volume type optical disk system expected as a next-generation optical disk system after the Blu-ray optical disk system and also as a light source demanded from the medical field, bio-imaging field and the like.
A titanium/sapphire laser, for example, is known as an ultrashort pulse/ultrahigh output laser, but the titanium/sapphire laser is an expensive large solid-state laser light source, which constitutes a main factor inhibiting widespread use of the technology. Realization of the ultrashort pulse/ultrahigh output laser by a semiconductor laser or a semiconductor laser device will cause significant miniaturization, cost reduction, and high-level stabilization and is considered to be a breakthrough to promote widespread use thereof in these fields.
On the other hand, research on shorter pulses of a semiconductor laser device has actively been done in the field of communications systems since the 1960s. The gain switching, the loss switching (Q switching), and the mode locking are known as methods of causing a semiconductor laser device to generate shorter pulses and these methods aim for higher output by combining the semiconductor laser device with a semiconductor amplifier, nonlinear optical element, optical fiber or the like. The mode locking is further divided into active mode locking and passive mode locking. To generate an optical pulse based on the active mode locking, an external resonator is constructed using mirrors and lenses and further radio frequency (RF) modulation is applied to the semiconductor laser device. For the passive mode locking, on the other hand, an optical pulse may be generated by simple DC driving using a semiconductor laser device having a multi-electrode structure.
It is necessary to provide emission regions and saturable absorption regions in a semiconductor laser device to cause a self-pulsation operation of the semiconductor laser device. Based on an arrangement state of emission regions and saturable absorption regions, the semiconductor laser device may be classified into a SAL (Saturable Absorber Layer) type or WI (Weakly Index guide) type in which emission regions and saturable absorption regions are arranged in a vertical direction and a multi-electrode type including a bi-section type in which emission regions and saturable absorption regions are arranged in a resonator direction. A bi-section type semiconductor laser device is known from Japanese Patent Application Laid-Open Nos. 2004-007002, 2004-188678, and 2008-047692. Compared with a SAL type semiconductor laser device, a multi-electrode type GaN base semiconductor laser device has a larger effect of saturable absorption and is considered to be able to generate optical pulses whose width is narrow.